The spine is made up of twenty-four vertebrae that are stacked one upon the other to form the spinal column. The spine provides strength and support to allow the body to stand and to move with some flexibility. Each vertebra includes an opening through which the spinal cord passes. The collection of vertebrae thus protects the spinal cord. The spinal cord includes thirty-one pairs of nerve roots that branch from either side of the spinal cord off to other parts of the body. Those nerve roots extend through spaces between the vertebrae or in the vertebrae known as the neural foramen.
Between each pair of the adjacent vertebrae is an intervertebral disc. The disc is made up of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral endplates. The disc serves several purposes, including absorbing shock, relieving friction, and handling pressure exerted between the superior and inferior vertebral bodies associated with the disc. The disc also absorbs stress between the vertebral bodies, which stress would otherwise lead to degeneration or fracture of the vertebral bodies.
Disorders of the spine are some of the costliest and most debilitating health problems facing the populations of the United States and the rest of the world, costing billions of dollars each year. Moreover, as the population continues to age, the incidence of spinal disorders continues to grow, including those caused by disease, trauma, genetic disorders, and other causes.
Spine disorders are treated in a number of different ways. Medicinal treatments, exercise, and physical therapy are typical conservative treatment options. Less conservative treatment options include surgeries, such as microdiscectomy, kyphoplasty, laminectomy, dynamic stabilization, disc arthroplasty, and spinal fusion. Traditionally, these treatment options are used in isolation, rather than in combination, and the most conservative of the treatment options utilized to provide a desired result.
U.S. patent application Ser. No. 11/281,205, entitled “Prosthetic Intervertebral Discs,” (“the '205 application”), was filed Nov. 15, 2005, was published as Publication 2007/0050033 on Mar. 1, 2007 and is assigned to Spinal Kinetics, Inc., the assignee of this application. The '205 application describes, inter alia, a treatment option that combines a prosthetic intervertebral disc with a dynamic stabilization system. The '205 application (and its provisional predecessor Ser. No. 60/713,671, filed Sep. 1, 2005) are incorporated by reference.
In 1992, Dr. Manohar Panjabi introduced a model of a dynamic spinal stabilization system that describes the interaction between structures that stabilize the spine and defined spinal instability as a region of laxity around the neutral resting position of a spinal segment, identified as the “neutral zone.” Panjabi, M M., “The Stabilizing System of the Spine. Part II. Neutral Zone and Instability Hypothesis.” J Spinal Disord 5 (4): 390-397, 1992b. There is some evidence that the breadth of the neutral zone increases as a result of intervertebral disc degeneration, spinal injury, and spinal fixation. Id. Panjabi has subsequently described dynamic stabilization systems that provide increased mechanical support in the neutral zone and decreased support away from the neutral zone. See, US Patent Publication No. 2004/0236329, dated Nov. 25, 2004, which is hereby incorporated by reference herein.